The invention relates generally to vehicle braking systems of the type having a control system in the form of an air supply modulated by the brake pedal for controlling an hydraulic braking system, with the two systems being coupled together by an intensifier in which an air pressure signal is converted into a higher hydraulic pressure in accordance with a predetermined intensification ratio.
A typical intensifier unit consists of a housing having a large low pressure cylinder with an air piston and a relatively small high pressure cylinder with an oil piston, the two pistons being connected together so that the hydraulic pressure at an outlet connection varies in accordance with the air pressure applied to an inlet connection. Prior devices have been relatively complicated and expensive requiring adherence to close tolerances and utilizing relatively complicated sealing and valving arrangements to isolate the two fluids. Prior intensifier units have, moreover, been limited to a single predetermined intensification ratio so that accommodating the needs of a wide range of vehicles has required a catalog series of units in incremental sizes. Moreover, prior devices have generally employed a moving seal carried by the high pressure piston, with replenishment of the hydraulic fluid, after the brakes are released, occurring via a series of small bleed holes which lie just ahead of such piston when the latter is in reference position. In such constructions it is necessary for the sealing element, carried by the piston, to traverse the bleed holes during each stroke. If the bleed holes are made of large diameter they affect the integrity of the surface and cause aggravated wear of the sealing element. On the other hand if the bleed holes are made of small diameter they restrict flow of the replenishing fluid, particularly at low temperatures, making it necessary to limit use to a fluid of low viscosity. Low viscosity fluids tend to exhibit high vapor pressures, thereby causing vapor pockets when the temperature subsequently rises causing the braking action to become "mushy" and creating a possibly hazardous condition. Moreover, use of small bleed openings invites clogging by any solid particles which may be carried by the hydraulic fluid particularly in the case of poorly maintained systems which have been operated for long periods of time.
In another popular design of intensifier there are no fixed bleed ports, as such, and replenishment occurs via special valving built into the hydraulic piston. Provision of such valving adds greatly to the expense and complexity and gives rise to service problems.
In conventional intensifiers, also, it has been the practice to use a short hydraulic piston connected to a relatively small diameter piston rod. As the piston moves forwardly, hydraulic fluid is sucked in to fill the annular void behind the piston and, upon the return stroke, such oil is expelled. This results in idle pumping of fluid in and out during each cycle resulting in unnecessary agitation of the fluid, possible cavitation with resultant leakage, and the risk of pumping harmful impurities into the unit at each stroke. For accommodating the fluid flowing back and forth a relatively large fluid reservoir must be employed and the line leading to the reservoir, as well as the fittings, must be of large size to handle a quantity of fluid many times greater than that required for make-up purposes in the brake system.